The present invention relates generally to a printing machine, and more particularly to a printing machine having a novel ink dispensing arrangement.
The present invention is particularly but not exclusively applicable to screen printing machines where it is highly advantageous. However, it is generally applicable wherever it is necessary to dispense liquid or pasty media, such as printing ink and the like.
Particularly screen printing machines have become very widely popular for printing of fabrics, such as woven and knitted fabrics, fibrous webs, paper, synthetic plastic foil, carpets, rugs and the like. In the case of screen printing machines the squeegee usually performs the dispensing function for dispensing the printing ink, that is for causing the printing ink to be squeezed through the openings in the printing screen. There are so-called roller squeegees which are rotated by friction, being located in the interior of a tubular printing screen and squeezing the printing ink through the perforations of the printing screen. There are also other types of squeegees which serve to wipe or otherwise force the printing ink through the perforations of the printing screen.
It is well known in the art that the depth to which the ink that has been dispensed through the printing screen, will penetrate into the workpiece, depends to a large extent upon the characteristics of the workpiece itself. In other words, the question of how much and how readily the workpiece will absorb liquid or a pasty printing ink is of great importance in determining the depth of penetration of the printing ink into the workpiece. Of course, the consistency of the printing ink in turn is of importance, because the more liquid the printing ink, the more readily it will penetrate into the workpiece. Also important is the period of time during which the material is afforded the possibility of penetrating, that is the speed at which any given portion of the workpiece moves out of the region in which printing ink penetrates into it through the printing screen. In many instances these three factors cannot be varied to suit particular requirements. The type of material for the workpiece is usually predetermined by the requirements of a particular job, so that its characteristics cannot be adapted to the printing requirements. Conversely, the consistency of the printing medium also cannot, as a rule, be varied because certain characteristics and consistencies are required of the ink. The third factor, namely the operating speed of the printing machine, also cannot usually be varied since a certain throughput per unit of time is required of the machine in order to obtain economic production.
This leaves a fourth factor which can sometimes be varied within a certain range, namely the level of the media sump from which the medium -- such as the printing ink -- passes through the printing stream. Theoretically, the higher the level of the sump, and therefore the greater the inherent weight of the medium that tends to force the medium adjacent the printing screen through the perforations of the latter, the greater the pressure at which the medium will be forced through these perforations. However, there are definite and relatively narrow limits within which variations can be made in this factor, because the level to which the sump can be built up can only be increased to a certain extent.